1. Field of the Invention
Fuel injection systems incorporating fuel recirculation may require increased maintenance as a result of degradation of rubber stock, e.g., hoses, which are contacted by the fuel. This problem has been attributed to an attack on the rubber elastomer by oxidation products present in the fuel, which attack results in a loss of physical properties and ultimately a failure of the vulcanizate. This problem is aggravated by the fact that unlike in the case of conventional fuel systems, fuel is delivered to the engine under pressure (0.2-0.3 MPa) in fuel injection systems.
While sour fuel contains hydroperoxides which are the result of oxidation of the fuel, these peroxides themselves appear to have little affect on the durability of the rubber stock. However, in the presence of metal ions, like copper, hydroperoxides can catalytically decompose into free radicals, which then can attack the elastomer. Metal ions present in the fuel not only catalyze the formation of the radicals but also promote the initial formation of the hydroperoxides in the fuel. Although trace amounts of metals, such as copper, may be present naturally in the fuel, the use of the fuel injection system increases the metal ion concentration in the fuel in several ways. First, since in a fuel injection system the fuel pump is submerged in the fuel tank and has a copper commutator which is cooled by the fuel passing over it, copper ions and metal may be added to the fuel through frictional wear and electrical arcing. Second, since in this type of system, a majority of the fuel is recirculated to the fuel tank, unlike conventional carburetor systems, metal ions such as copper can build up in the fuel tank. The increased presence of these metals in the fuel accelerates the oxidation whereby hydroperoxides are formed and subsequently converted to free radicals.
The effect the free radical has on elastomers varies. In the case of commonly used epichlorohydrin elastomers, for example, the compound softens due to chain scissioning. Nitrile elastomers (NBR), on the other hand, harden in the presence of sour fuel. In NBR this is attributed to an increase in the number of crosslinks. In both epichlorohydrin and NBR, the free radical attacks reactive sites on the polymer backbone.
We have now found that a particular urethane cured nitrile elastomer vulcanizate possesses excellent resistance to sour fuel degradation, making it most suitable for use in fuel systems wherein a vulcanizate with resistance to sour fuel degradation is desired.
2. Description of the Prior Art
Baker et al. in U.S. Pat. No. 3,645,980 teach a method of crosslinking natural or synthetic unsaturated rubber which comprises a urethane cure system much like that of this invention. The vulcanizate so produced is taught to be virtually reversion resistant, i.e., thermally stable and resistant to oxidation, when compared to conventionally sulfur cured vulcanizates. In their subsequent paper "Urethane Crosslinking of Natural Rubber", International Rubber Conference, P. G2 through G2-8 (1972), Baker et al. emphasize that the usefulness of this urethane cure system is directed primarily to developing natural rubber vulcanizates which are capable of long service times at elevated temperatures. Self admittedly, Baker et al did little work on synthetic rubber, such as the nitrile rubber of the subject invention, teaching that although the urethane reagents will cure the synthetic rubbers and give excellent reversion resistance, the sulfur-cured synthetic rubbers already had adequate reversion resistant for the applications in which they are employed, thereby making the urethane cure less useful than in the case of natural rubber.
We have now found, in contrast to the suggestion in Baker et al, that the thermal and oxidative resistance of urethane cured nitrile vulcanizate at elevated temperatures above 140.degree. C., are in fact, inferior to those of sulfur cured nitrile vulcanizates. Based on knowledge of the similarity of mechanism involved (oxidation) in such high temperature degradation and degradation in the presence of sour fuel, one skilled in the art would have expected the urethane cured nitrile vulcanizate to exhibit poor sour fuel resistance. However, quite unexpectedly, it has been found that such urethane cured nitrile vulcanizates do demonstrate excellent resistance to sour fuel, thus making them an excellent selection for use in fuel system environments which may be exposed to sour fuel.
It is an object of this invention to provide an article adapted for use in sour fuel environment comprising a vulcanizate cured by urethane or urethane/sulfur and exhibiting resistance to sour fuel degradation.